1. Field of the Invention
The present invention relates to lithium ion batteries, and more particularly to lithium ion batteries comprising nanomaterials.
2. Description of Prior Art
Lithium ion batteries are popularly used because of their high capacity and environmentally friendly features. These batteries are becoming of increasing importance for a wide variety of uses, ranging from portable electronics to power supply devices for spacecraft. There is an ongoing need for advanced lithium ion batteries with higher capacity.
A conventional lithium ion battery comprises an anode, a cathode, an electrolyte, and a membrane. The membrane separates the anode from the cathode. The membrane is porous, allowing ions to pass therethrough while blocking electrons from passing therethrough. The membrane can be made from polyethylene, polypropylene or polystyrene. The cathode is generally made of lithium (Li) doped transition metal oxides such as LiMO2 (M=cobalt, nickel or manganese) that function as electron donors. Atoms of the electron donor undergo an oxidation reaction to form ions of lithium, and free electrons. These ions travel through the membrane and are absorbed by the anode by a reduction reaction, and the free electrons travel through an electrical circuit to provide electrical current. The anode is generally made of materials comprising carbon, such as graphite or carbon fibers. The carbon materials can readily intercalate lithium ions. Since the number of electrons “given away” by each atom of the electron donor is known, by measuring the number of electrons transferred through the electrical circuit, the number of ions transferred can be determined. This quantity is related to the specific capacity of the battery, which can be expressed in milliampere-hours per gram of the material. For example, the maximum specific (reversible) capacity of graphite to accept lithium ions is reported to be approximately 372 mAh/g, characterized as (LiC6) in an article by J. R. Dahn et al. entitled “Mechanisms for Lithium Insertion in Carbonaceous Materials” (Science, Volume 270, Oct. 27, 1995).
Other carbonaceous materials are used as electrodes in advanced lithium ion batteries. Carbon nanotubes are considered to be promising candidates for electrode materials. These nanotubes can be visualized as “rolled-up” sheets of carbon hexagons, providing a superior intercalation medium for lithium ion batteries. The intercalation capacity of single-walled carbon nanotubes has been reported by Jijun Zhao et al. in an article entitled “First-Principles Study of Li-Intercalated Carbon Nanotube Ropes” (Physical Review Letters, Vol. 85, P1706˜1709, 2000). Results of the study show that, energetically, the insides of carbon nanotubes are as favorable as interstitial sites between carbon nanotubes for intercalation of lithium ions, and that the intercalation potential of single-walled carbon nanotubes is similar to that of graphite. Further, the density of the intercalated lithium ions in single-walled carbon nanotubes is higher than that in graphite, being up to about Li0.5C.
U.S. Pat. No. 6,280,697 discloses a lithium ion battery employing single-walled carbon nanotubes as an electrode, and a method for making the electrode. The method comprises the following steps: preparing single-walled carbon nanotubes by laser ablation; washing and purifying the single-walled carbon nanotubes to obtain a purified material containing over 80% by volume of single-walled carbon nanotubes; placing the purified carbon nanotubes in a suitable solvent to form a solution; immersing a suitable substrate in the solution to deposit a film having the single-walled carbon nanotubes therein on the substrate; and driving off the solvent to leave the film of the single-walled carbon nanotubes covering at least an upper swface of the substrate and to promote adhesion of the film to the substrate. The substrate with the single-walled carbon nanotubes deposited thereon can then be used as an electrode of a lithium ion battery. However, the laser ablation method does not provide for satisfactory control of the diameter, length or direction of growth of the single-walled carbon nanotubes formed, and the yield of this method is relatively low. Moreover, excess amorphous carbon nanoparticles and metal catalysts are produced along with the single-walled carbon nanotubes, thus necessitating complicated purification processes. In summary, industrial production of single-walled carbon nanotubes using this method is problematic, and the capacity of the lithium ion battery employing the single-walled carbon nanotubes as electrodes formed by this method is limited.